1. Field of the Invention
This invention relates to a video signal processing device and more particularly to a video signal processing device which is arranged to perform an emphasizing process for the magnetic recording of a video signal and a de-emphasizing process for reproduction of a magnetically recorded video signal.
2. Description of the Prior Art
In recording and reproducing a video signal on a magnetic recording tape or the like through frequency modulation by use of a VTR, for example, heretofore the deterioration of the S/N ratio of reproduction picture signal due to noises generated on the magnetic recording tape or the like has been mitigated by emphasizing the high frequency component of the video signal through an emphasis circuit in recording the video signal and by reducing and suppressing the high frequency component through a de-emphasis circuit in reproducing the recording.
In the case of a home VTR in general, a circuit which removes a low level signal portion of the high frequency component of a video signal, has been used.
Referring to FIGS. 1 to 3 of the accompanying drawings, examples of these conventional circuits are as described below:
FIG. 1 shows an example of the conventional circuit for emphasizing in recording a video signal. This example comprises an emphasis circuit consisting of a capacitor C and resistors R and a clipping circuit 2 which is provided for clipping a black and white signal which has the high frequency component thereof emphasized through the emphasis circuit. FIG. 2 shows the conventional circuit arrangement used to de-emphasize a reproduction signal and to perform the noise clipping action. In FIG. 2, the circuit arrangement comprises a de-emphasis circuit 3 which consists of a capacitor C and resistors R; a buffer amplifier 4 which is connected to the rear stage of the de-emphasis circuit 3; a subtractor 5 which is provided for to subtract the value of the output of the buffer amplifier 4 obtained through a high-pass filter 6 and an amplitude limiter 7 from a value of the output of the de-emphasis circuit 3 obtained through the buffer amplifier 4. A noise clipping circuit NC is formed by the subtracter 5, the high-pass filter 6 and the amplitude limiter 7.
The operation of the conventional circuits shown in FIGS. 1 and 2 is as described below with reference to the wave form chart of FIG. 3.
First, an emphasizing action is performed in the following manner: In the emphasis circuit 1, a high frequency emphasizing filter is formed by the capacitor and resistor, C and R. Accordingly, where an input video signal Sa is a signal of a wave form as represented by FIG. 3(A), for example, the output of this emphasis circuit 1 becomes a signal Sb of a wave form as represented by FIG. 3(B) in which overshooting takes place at the rise and fall parts thereof. In the case of a home VTR, a fairly large overshoot peak results from emphasizing a signal of at least about 1 MHz by about 12 to 16 dB. Therefore, frequency modulation of such a signal as it is would result in over-modulation. In such a case there takes place an inversive phenomenon, since the recording band of the tape transport system of a magnetic head is limited to prevent reproduction of high frequency. To prevent this, the clipping circuit 2 for limiting the amplitude of overshooting to obtain a wave form as represented by Sc in FIG. 3(C) before frequency modulation is provided. After that, the signal is recorded on a magnetic tape by a magnetic head.
In reproducing a recording made by a VTR, a signal recorded on a magnetic tape is read out by a magnetic head and is amplified. The amplified signal is then frequency demodulated. The demodulated signal is supplied to the de-emphasis circuit 3. Circuit 3 is a low-pass filter of opposite characteristics to that of the emphasis circuit 1 shown in FIG. 1. The output Sd of the de-emphasis circuit which is of a wave form as shown in FIG. 3(D) becomes about the same as the original signal with the exception of the part limited by the clipping circuit 2. At this time, the components of noises which have been generated by the magnetic tape, magnetic head, etc. and have been distributed within the high frequency zone of the video signal are decreased by the low-pass characteristic of the de-emphasis circuit 3.
Recording with a VTR, however, particularly with a home VTR, is accomplished at a high degree of density, therefore, the S/N ratio of a video signal is insufficient. Hence, the S/N ratio is improved by the provision of the noise clipping circuit NC shown in FIG. 2.
Further, at this time, since the high frequency component at the rise and fall of the video signal has been lost there, the signal Sd shown in FIG. 3(D) is of a wave form in which smearing has occurred to deteriorate the quality of a picture. Further, the white peak portion of the recording signal Sc corresponds to a short wave length portion of a poor S/N ratio in an ordinary magnetic recording tape, the S/N ratio of the reproduced signal Sd is deteriorated for a period of about several .mu.sec after the rise thereof. This results in a poor S/N ratio at the contour portion of a picture, an undesirable result.
The noise clipping circuit NC shown in FIG. 2 operates as follows: The demodulated, de-emphasized signal Sd is amplified at the buffer amplifier 4 and then passes to the high-pass filter 6 and to the subtracter 5. At the high-pass filter 6, which is normally formed by a capacitor and a resistor, the high frequency component of a video signal Se of a wave form as represented by FIG. 3(E) is taken out. This signal Se is passed through the amplitude limiter 7 to clip the video component to a level which has larger amplitude than the noises. The signal is then supplied to one of the input terminals of the subtracter 5 and is subtracted from the above-stated signal Sd. This offsets the noise component that is not clipped by the amplitude limiter 7. However, at the rise and fall parts of the video signal the output of the amplitude limiter 6, which is in a wave form as shown in FIG. 3(F), the noise component does not appear for a period of several .mu.sec after the rise and fall, therefore, the noise of these parts remains unremoved.
With the conventional emphasis circuit, de-emphasis circuit and noise clipping circuit, there smear appears at the rise and fall parts of a picture as shown in the output of the subtracter 5 represented by FIG. 3(G). Another shortcoming of the conventional circuits is that the S/N ratio is deteriorated for several .mu.sec after the rise and fall. In the conventional home VTR, the above-stated shortcoming has been prevented from actually causing a serious problem by giving some allowance to the S/N ratio of the magnetic system thereof by arranging the recording density to have an ample margin. However, recent demands for recording over a long period of time have compelled use of high density recording. Accordingly, the S/N ratio is no longer sufficient. Particularly, the S/N ratio at the contour portions of pictures has impeded the carrying out of high density recording.